In various fields, including telecommunications and telephony, there is need for amplifiers and/or transducer drivers with minimal current requirements that can deliver low distortion signals into relatively low impedance loads. This need is also present in other industries such as those involved with hearing-aids.
Transistor amplifiers may be classified according to the operating point of their active transistors. For a sinusoidal input, the active element in a so-called "Class A" amplifier conducts for the entire input cycle. In "Class B" amplifiers, the transistor is biased at cutoff, conducting for only half of each input cycle. "Class AB" amplifiers are in between Class A and Class B amplifiers, with collector current in the active transistor flowing for less than a full input cycle, but more than a half cycle. With Class AB operation, one transistor amplifies the positive part of an input signal, and another transistor amplifies the negative part of the input signal. A transistor in "Class C" operation is biased well into its cutoff region, a technique used mainly in radio-frequency tuned amplifiers.
A known problem with Class B amplifiers is crossover distortion, a phenomenon which arises from the curvature of a transistor's input characteristic at very low current levels. Due to this curvature, a transistor operating in Class B and biased at cutoff (i.e. with zero collector current) will tend to distort low-level signals. In practice, crossover distortion in a Class B amplifier manifests itself as kinks in the amplifier's output characteristics in the crossover region where one of the Class B transistors is turning on and the other Class B transistor is turning off.
One way of reducing problems with crossover distortion is to bias each stage of the Class B amplifier above cutoff, outside of the curved region of each transistor's input characteristic. This technique, however, increases the current consumption of the amplifier, since a DC current is present even when the input signal is zero. This technique essentially results in Class AB operation, even though such amplifiers may still be referred to as Class B.